It is useful to analyze the backside of a sample, such as a semiconductor wafer, at various stages in fabrication and/or testing of semiconductor devices. One technique for obtaining high resolution images of a wafer backside is to physically flip over the wafer and collect images utilizing a state of the art review system. However, doing so can destroy front side features of the wafer. This makes backside review impractical for a wafer that is nearly complete and potentially still yielding die.
Review images are typically collected utilizing a Bayer filter collection scheme. Red, green, and blue filters are placed above each pixel to create a color checkerboard. Then a color image can be reconstructed by interpolating red, green, and blue values for each point. A second method, generally accepted to give superior images to the Bayer camera, is a “3-chip camera”. These cameras incorporate means for color separation, such as a prism, at the input of the camera and split red, green, and blue illumination to 3 separate focal planes. Both of these prior methods work with “white light” illumination (e.g. broadband halogen bulbs or white LEDs) to take RGB images.
The Bayer filter camera setup can result in “checkerboard” artifacts caused by interpolation of the image needed to fill in the full image in RGB color. The interpolation error can be minimized by designing for more pixels over the optical resolution (or Airy pattern)—but doing so increases the pixel count of the camera or results in a decreased field of view. The 3-chip camera is unattractive for size, cost, and optical aberrations caused by the color separation. The aberrations may include one or more of: astigmatism due to the air gap in the separation prism, TIR loss “striping features” due to the air gap in the prism, relative focus errors due to manufacturing tolerances for Z position of the chips, and variations in distortion due to prism manufacturing tolerances that cause lateral color features that vary between colors.